A direct-transfer polymerization model explains how the multiple profilin-binding sites in the actoclampin motor promote rapid actin-based motility

The high actin-based motility rates observed in nonmuscle cells require the per-second addition of 400–500 monomers to the barbed ends of growing actin filaments. The chief polymerization-competent species is profilin · actin · ATP (present at 5–40 μM intracellular concentrations), whereas G-actin · ATP is much less abundant (∼0.1–1 μM). While earlier studies unambiguously demonstrated that profilin · actin is highly concentrated within the polymerization zone, profilin-actin localization on the motile surface cannot increase the local solution-phase concentration of polymerizable actin. To explain these high rates of actin polymerization, we present and analyze a novel polymerization model in which monomers are directly transferred to growing filament ends in the actoclampin motor. This direct-transfer polymerization mechanism endows the polymerization zone with properties unavailable to bulk-phase actin monomers, and our model also indicates why profilin is the ideal mobile carrier for actin monomers.